Is it OK to modify app/Mage.php to edit the date_default_timezone_set value?

I’ve modified my timezone in magento at System => General => Locale Options => Timezone, and set it to America/New_York, but it’s still showing UTC time and every time based function is keying off of UTC time.

When I look at app/Mage.php it shows:

if (is_readable($  localConfigFile)) {     libxml_disable_entity_loader(false);     $  localConfig = simplexml_load_file($  localConfigFile);     date_default_timezone_set('UTC');     if (($  date = $  localConfig->global->install->date) && strtotime($  date)) {         self::$  _isInstalled = true;     } } 

Thus, I assume I have to modify date_local_timezone_set from UTC to America/New_York? But is it OK to modify the app/Mage.php in this manner?

Thank you.